Hi everyone I am having some new issues with my 2008 iMac 20" 2.66ghz
The screen will occasionally turn to garbage (see attached) crashing the computer and sometimes it stays that way even after several resets. Other times the screen will turn right back to normal but the computer will stay locked up and need a hard reset. SSW is Lion and up to date although I'd love to downgrade to Snow Leopard to get Rosetta back.

Hello inzgary,
looks quite much like "dying" RAM-Module or harddisk.....
the fact that it is occasionally - might indicate a heavy problem with temperature caused by
nonoperating vents or bad cooling.....
the ventilation in the imac is probably affected and causes the overheated processor to crash....
it seems the graphicprocessor or the mainprocessor is getting toasted slowly....
you should not continue unless thats repaired -unless you want the taost to become finished....
temperature is a common problem in the imacs, specially the laptops - and you will find quite a lot hits in the internet....
some of them have good description of DIY-repairs....
sincerely speedyG

I get a similar view to yours, but with a lot more red, on one the DVI ports on my PC. There's a problem somewhere close to the video port that I haven't been able to track down. I'm reluctant to do anything about it, since the other ports work, and it's otherwise a good card.

If you don't get any satisfaction from checking RAM and dust-bunny removal, I'd suggest looking at the video subsystem. Are you able to test video on an external monitor?

I get a similar view to yours, but with a lot more red, on one the DVI ports on my PC. There's a problem somewhere close to the video port that I haven't been able to track down. I'm reluctant to do anything about it, since the other ports work, and it's otherwise a good card.

If you don't get any satisfaction from checking RAM and dust-bunny removal, I'd suggest looking at the video subsystem. Are you able to test video on an external monitor?

Hello eeun,
in comparision with the mistake you explained - there are several differences:first difference: your computer enters working mode and does not freezesecond difference: your display only displays with one missing colorthird difference ( guessed by me ): the mistakes happen occasionelly - i even go one step ahead and believe that one point has not been mentioned... : there probably is a dependence between time of computer startup and mistake:
the more the starts are tried the faster the mistakes happen.... this would be proof that after mistake happens there is the need to first cooldown the system before there is a minor chance that system gets up and then after heating upfreezes again....

at least the first two differences show very clear that your problem is located within the path between the displaycircuits and the monitor ( including the cable and plugs as possible errorpossibilities )
while on the other hand the other issue discussed here is clearly located within the system itself ( indicated by the freezes ) .......
so you are talking about a clearly other not comparable problem.....
sincerely speedyG

Having repaired several Imacs that all had the same problem, I'd suggest you inspect the electrolytic caps in the power supply and main board.
If any are bulging or have blown, that's the likely problem.
Keep in mind that replacing them isn't always easy.
It can take a LOT of heat to melt the solder Apple used.
I have many temperature controlled Weller irons and needed to use a 100/150w soldering gun to melt their solder.
(remove the harddrive if you do this. Soldering guns emit powerful magnetic fields.)

Also on one Imac I noticed that the heatsink fastening clip on the CPU had broken and the heatsink wasn't fully on the chip.

...which is why I inferred he should first try the RAM and check the cooling as you'd suggested.
Remote-diagnosing systems based on someone's post online is difficult, and there's no harm in offering another option if the first one doesn't work. I also agree with the subsequent post of checking the board's caps as another option.

I have also had video cards that would appear to lock up the system as they overheated, so it's not "clearly" an option to completely dismiss. And yes, I've checked the cable. :/

I didn't mean to imply that the video card wasn't overheating or there was any other problem(s) present.
On my own Imac, the video card did the same thing.
However the reasons cards heat up more where they didn't previously is usually due to increased AC ripple (bad caps) or dust.
Before anyone attempts a component level repair, I wholeheartedly suggest they open their unit and blow out the dust.

Also, I was able to obtain a little longer running time before lockups by installing smcFanControl.
With it, I was able to boost the airflow and keep things cooler.
Of course boosting fan speed also boosts current draw which increases heat and destroys caps, but at least in my case it helped buy a little time.

Thanks everyone for the replys. Well I installed smcfancontrol and cranked the fans up. The hottest the cpu read was 120°F and it was fine for a couple days. Now it has taken a turn for the worst and the problem is constant. Single user mode does not crash even though there is garbage on screen 99% of the time. Ram testing revealed no issues...so I think this means a failed cap?

Hello inzgary,if the mistake turns to become constant and its not dependent to temperature the option points to the components itsself - and among the components themselves the electrolytic capacitors are the first choice for hunting.

There are 2 ways to hit the trouble:first to inspect the circuitboard very carefully hunting for spots with greasy looking film ( i.e. when caps start leaking and the bor-acid leaks out ) or if top of caps turned from flat top to rounded hoods or other unusual appearance..... in such case those caps should be replaced....

or the second "wide-shot" if the related caps can´t be spotted out by appearanceto exchange on doubt all caps in the related videoarea and along the powerlines ( thats a really timeeating and expensive task - so a careful inspection might save some money and time - but sometimes the caps really die without a change in the appearance and the "wideshot" can´t be avoided.

In case of replacement of the caps there is one rule : if value can´t be replaced exactly the replacement cap should never have smaller values than the original one - and always ensure to keep correct orientation of polarity - otherwise they might blast like firecrackers ! In case of doubt mark the orientation at the PCB with a markerpen !

Capacitorvalues are displayed by : value/voltage (i.e. 47µF /16V = 47 microfarady/ 16 Volt volt Limit ) so such a cap may be replaced with a cap like 68µF / 16 Volt or 47µF / 25 Volt. ... but its best to keep up with the matching same values ! Some companies mark only the positive pol with a "+" sign and some companies mark the negative pol by a kind of line on one side.

What I did find when I took it apart was the large heat sink was clogged with dust. All the caps on the board were solid and saw no bulging tops even on the paltry 180w power supply. Since the heatsink cools both the GPU and CPU one of those is probably heat damaged but both look removable. I'm surprised there is no emergency shutoff to prevent overheating like previous imacs. I'll see if clearing the heat sink stabilizes the computer and if not start with the AMD card.

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